1. Field of the Invention
The present invention relates to a magnetic rotor and a rotational angle sensing apparatus having the same.
2. Description of Related Art
A previously known rotational angle sensing apparatus includes a magnet and a plurality of magnetic sensing elements. The magnet is installed around a rotatable shaft. The magnetic sensing elements output measurement signals of different sine waveforms, which have different phases, according to a change in a magnetic field of the magnet caused by rotation of the rotatable shaft. In such a previously known rotational angle sensing apparatus, an absolute rotational angle of a sensing subject can be sensed (or determined) based on the measurement signals of the magnetic sensing elements.
However, in reality, the waveform of the measurement single of each magnetic sensing element is not a precise sine waveform for the following reason. Specifically, in the rotational angle sensing apparatus, which senses the absolute rotational angle (hereinafter, simply referred as “rotational angle”), it is required to accurately position the magnet relative to the rotatable shaft in the circumferential direction. In view of this requirement, in a magnetic rotor 520, which includes a magnet 524 and a rotatable shaft 512 shown in FIG. 11, a key 528 is fitted into a key groove 524a, which is formed in an inner peripheral wall of the magnet 524, and a key groove 526a, which is formed in an outer peripheral wall of a yoke 526, so that the magnet 524 is positioned relative to the rotatable shaft 512 in the circumferential direction (see, for example, Japanese Unexamined Patent Publication No. H11-332148). Thus, in such a magnetic rotor, a shape of the magnet is not symmetrical about a rotational axis of the rotatable shaft due to the fact that the key groove is formed in the magnet. Thus, the magnetic field of the magnetic rotor is not uniform, so that the waveform of the measurement single of the magnetic sensing element does not become the precise sine waveform. Therefore, the measurement error of the rotational angle sensing apparatus is increased.